drm/i915: Convert backlight_lock to a mutex

Originally the irq safe spinlock was required because of asle
interrupts. But since

commit 91a60f20712179e56b7a6c3d332a5f6f9a54aa11
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Thu Oct 31 18:55:48 2013 +0200

    drm/i915: move opregion asle request handling to a work queue

there's no need for this any more. So switch to the simpler mutex.

v2: Cite the right commit, spotted by Jani.

Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 17dfce0..07dafa2 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1528,7 +1528,7 @@
 	struct intel_overlay *overlay;
 
 	/* backlight registers and fields in struct intel_panel */
-	spinlock_t backlight_lock;
+	struct mutex backlight_lock;
 
 	/* LVDS info */
 	bool no_aux_handshake;